Method and apparatus for measuring nitrogen oxides and sulfur dioxideconcentrations

ABSTRACT

Nitrogen oxide and sulfur dioxide concentrations present in a gaseous mixture are rapidly and continually monitored by measuring the current passing between an inert metallic sensing electrode and a counter electrode which electrodes are in contact with an aqueous electrolyte solution and at which sensing electrode the oxides are electrooxidized. The sensing electrode is composed of an inert metal, whereas the counter electrode is composed of an electroactive lead sulfate which is electrochemically reduced when electrically interconnected with the sensing electrode in the presence of the aqueous electrolyte solution. A variable voltage source maintains the potential at the sensing electrode at the desired level.

United States Patent 1111 3,763,025

Chand Oct. 2 1973 l l METHOD AND APPARATUS FOR MEASURING NITROGEN OXIDES AND Primary Examiner-T. Tung SULFUR DIOXIDE CONCENTRATlONS AlmmyLcwis Dalgar" [75] Inventor: Ramesh Chand, Simi Valley, Calif.

[57] ABSTRACT [73] Asslgnee: Dynasciences Corporation, Los

Angeles Calif Nltrogen mode and sulfur dioxide concentrations present in a gaseous mixture are rapidly and continually [22] Filed: 1971 monitored by measuring the current passing between [2i] Appl. No.: 200,870 an inert metallic sensing electrode and a counter electrode which electrodes are in contact with an aqueous electrolyte solution and at which sensing electrode the [52] 204/1 204/195 204/195 P oxides are electrooxidized. The: sensing electrode is [5 Int. Cl. i composed of an inert metal whereas the counter elec- [58] Field of Search 204/] T, 195 R, 195 P trodg is composed f an electmactive lead lf t 1 1 which is electrochemically reduced when electrically Reierences cued interconnected with the sensing electrode in the pres- UNITED STATES PATENTS ence of the aqueous electrolyte solution A variable 3,429,796 2 1969 Lauer 204 195 P voltage Source maintains the P0tential at the Sensing FOREIGN PATENTS OR APPLICATIONS electmde at the des'red level 1,233,173 1/1967 Germany 204 195 R 5 Claims, 2 Drawing Figures [25 [I8 r24 r28 METHOD AND APPARATUS FOR MEASURING NITROGEN OXIDES AND SULFUR DIOXIDE CONCENTRATIONS BACKGROUND OF THE INVENTION The oxides of nitrogen have become a serious atmospheric pollutant especially in areas in which there is a relatively high density of automobiles. It has been found that the exhaust of internal combustion engines which burn hydrocarbon fuels such as gasoline expel rather high concentrations of these nitrogen oxides into the atmosphere, and due to the catalytic action of sunlight, a number of rather complex compounds are formed. The presence of significant concentrations of these oxides causes serious problems of atmospheric pollution, commonly referred to as smog, especially in large metropolitan areas in which automobiles are a major mode of transportation. This atmospheric contamination is being more and more seriously regarded in attempts to check or otherwise reduce the serious effects caused by such atmospheric pollutants. It is well appreciated that the presence of such conditions as smog are extremely undesirable from a health standpoint, with eye and lung irritations being the most obvious problems. It is further known that this atmospheric contamination also inhibits and destroys plant growth as well. Aside from being one of the major contributors to smog, the oxides of nitrogen are themselves extremely dangerous and poisonous. Accordingly, the importance of being able to continuously monitor the concentrations of these compounds is readily evident.

Sulfur dioxide is also a serious atmospheric pollutant. According to the United States Public Health Service, amounts of up to twenty-five million tons per year are expelled into the atmosphere thereby making it second only to carbon monoxide as a major source of pollutant. Sulfur dioxide is known to be extremely dangerous in view of its corrosive and potential poisonous charac teristics. This gas causes irritation and inflammation of the conjunctiva of the eyes and also affects the upper respiratory tract and bronchi. The inhalation of sufficient quantities of sulfur dioxide may cause edema of the lungs or glottis and may result in respiratory paralysis. ln moist air or fogs, the gas combines with water to form sulfurous acid which is slowly oxidized into sulfuric acid. Concentrations of about less than one part per million (ppm) are believed to be injurious to plant life. While concentrations of 400-500 ppm may result in fatality, amounts between 50 and 100 ppm are considered to be the maximum permissible concentrations for exposures of 30 to 60 minutes.

Although a number of nitrogen oxide and sulfur dioxide analyzers are presently available, these instruments are less than satisfactory for a number of reasons. These analyzers incorporate coulometric, colorimetric, electrical and thermal conductivity as well as infrared and ultraviolet absorbance methods of analysis. Such instruments are generally of high cost and bulkiness, thereby inhibiting their widespread use and acceptance. In addition, many of these analyzers can only be operated by skilled technical personnel often requiring a number of steps in effecting the analysis as well as critical calibration procedures and handling of chemical solutions. Further, instrumental response is often slow.

Nitrogen dioxide concentrations may be measured by indirect electrolytic reaction in a coulometer in which iodide or bromide ions are oxidized to iodine or bromine respectively. The iodine or bromine is then cathodically reduced by controlled potential electrolysis with the resulting current being proportional to the nitrogen oxide present. However, this method of analysis is severely limited because of nonspecificity, continued maintenance requirements and poor response times. Ozone interferes with the reactions and must be removed by chemical filters which require frequent maintenance and/or replacement. Further, the iodide or bromide solutions require constant replenishment with storage problems of these corrosive solutions being undesirable. The slow response time of such a coulometer has been reported to be as high as 15 to 25 minutes for a response.

The most common method of analyzing for sulfur dioxide is colorimetric analysis which relies on absorption of the gas in a chemical reagent which thereby changes color and is photometrically measured. Some disadvantages include considerable maintenance, poor response and bulky apparatus. Reagent solutions must be stored and constantly pumped and samples require rigorous conditioning to remove interfering species. Response time of such instruments is of the order of 15 minutes for 90 percent response. The size and operation of the instrument is such as to discourage its use outside the laboratory.

Electrical and thermoconductivity analyzers, although requiring less maintenance, are highly nonspecific, and are only suited to the measurement of laboratory samples which have been conditioned to re-' move interfering species.

Perhaps the most widely used analyzers previously available were those utilizing photometric means in which the infrared and ultraviolet absorbance properties of sulfur dioxide or of nitrogen dioxide and nitric oxide are monitored. With proper filter selection, such an instrument can be made specific for sulfur dioxide or for the nitrogen oxides. However, the size of the analyzer makes it bulky to handle thereby discouraging its use as a portable type instrument. Further, samples must be carefully handled and continuously conditioned in order to eliminate foreign particles which settle out at the cell windows and thereby affect the sensitivity of the instrument. In addition, changes in light source intensity and detector tube sensitivity also affect the measurements thereby necessitating frequent calibration checks.

More recently, electrochemical cells have been developed for use in detecting these pollutants. These employ a relatively simple structure that provides selective monitoring of these pollutants with ease and accuracy, as contrasted with the problems and complications encountered with the previous devices. Generally, these cells consisted of an inert sensing electrode immersed in an acid electrolyte with a suitable counter electrode of a material, such as PbO capable of being electroreduced in conjunction with the oxidation of dissolved pollutants at the sensing electrode so that the current flowing between the electrodes could be measured to indicate the pollutant concentration in the gas sample. The sensing electrode had to be maintained at a positive oxidizing potential (relative to the standard hydrogen electrode) above that needed to oxidize the pollutant gas being monitored, but below that at which interfering gases might be oxidized. Accordingly, the choice of suitable counter electrode materials was limited to those like PbO which provided an electroreduction reaction at a positive potential. However, such materials might not prove chemically compatible or practical in certain applications where other components of the sample might interfere with the specific reduction reaction. Thus, a wider choice of suitable counter electrode materials, particularly those capable of producing a compatible reduction reaction at negative relative potentials, is desired.

SUMMARY OF THE INVENTION A confined volume of aqueous acid electrolyte provides an active surface for receiving molecules from a nitrogen oxide and/or sulfur dioxide gas sample which is to be measured for the concentration of the respective pollutants. A sensing electrode of chemically inert material, generally a noble metal or alloy, is immersed in the electrolyte adjacent to the active surface of the electrolyte. A lead sulfate mixture counter electrode, capable of being electroreduced to produce a neutral compound is immersed in the electrolyte space from the sensing electrode on the side opposite the active surface of the electrolyte. The sensing electrode and the counter electrode are externally coupled through a means for maintaining the sensing electrode at a predetermined positive potential above at least 0.17 volts for sulfur dioxide and above approximately 1.03 volts for nitrogen oxides relative to the standard hydrogen refer-.

ence electrode. The counter electrode is reduced in the electrolyte at a negative potential of about 0.36 volts relative to the standard hydrogen reference electrode. The gaseous sample is introduced at the surface of the electrolyte so that molecules of the gaseous sample are dissolved in electrolyte at a rate proportional to the concentration of the pollutant in the sample. The molecules dissolved in the electrolyte are electrooxidized upon contacting the sensing electrode to produce negatively charged ions. The counter electrode is electroreduced to produce lead and negatively charged sulfate ions. The flow of current between the sensing electrode and counter electrode is measured to indicate the concentrations of the pollutants in the sample.

BRIEF DESCRIPTION OF THEDRAWING FIG. 1 illustrates a schematic cross-section of the transducer of the invention; and

FIG. 2 shows an enlarged detailed view of a broken away portion of the sensing electrode area of FIG. 1.

DESCRIPTION OF THE INVENTION It is to the elimination of the problems generally associated with the above-noted methods of analyzing for nitrogen oxides and sulfur dioxide that the present invention is directed. The most important nitrogen oxides present in the atmosphere are nitric oxide and nitrogen dioxide and it is to the analysis of these oxides, along with sulfur dioxide, that the invention is generally directed. However, it will be appreciated that other oxides, such as nitrous oxide, may also be monitored by proper selection of electrolyte and bias voltage within the scope of the disclosure.

This invention will be described primarily with reference with a nitrogen oxide containing sample, but it will be understood that the same process and apparatus is employed with regard to detecting concentrations of sulfur dioxide, except where specifically noted otherwise.

Specifically, the apparatus of the invention for measuring nitrogen dioxide and nitric oxide (and sulfur dioxide) concentrations present in a gaseous mixture comprises a transducer having a sensing electrode and 5 a counter electrode which contact an aqueous electrolyte solution. The transducer is constructed in such a manner that the sensing electrode, at which the nitrogen oxides are electrooxidized is located between the surface of the electrolyte which is in contact with the oxide containing atmosphere and the counter electrode. In this manner, as the gaseous mixture is diffused into the electrolyte at the gas-electrolyte interface, the nitrogen oxide (or sulfur dioxide) molecules will contact the sensing electrode and thereupon be electrooxidized to nitrate ions or sulfate ions respectively. At the same time, the counter electrode, which is in contact with the electrolyte solution and interconnected with the sensing electrode so as to allow a current to flow therebetween, is, electroreduced with the electrons given up during oxidation at the sensing electrode to produce the negative ions.

Initially, the operation of the transducer of the invention will be more readily appreciated and understood by referring to the schematic representation of the device as shown in the acompanying drawings.

FIG. 1 shows a schematic cross-section of a preferred construction of the transducer utilized in the invention whereas FIG. 2 illustrates, in detail, a portion of the device at the sensing electrode. Each of these drawings may be referred to in the following description. The electrolyte solution 11 is contained in a suitable vessel in which is present a counter electrode 12 and a sensing electrode 13. The vessel 10 may consist of any suitable material such as plastic, glass, etc. which is preferably impact resistant and shatterproof. A means for passing a current between the sensing and counter electrodes comprises a conductive wire 14. The wire 14 is attached to terminals 22 and 23 located on the exterior of the vessel 10 with the terminals 22 and 23 being interconnected to the respective electrodes via conduits 19 and 20. Voltage source 30 and variable resistor or potentiometer 32 in the sensing circuit provide a bias voltage between the two electrodes 12 and 13 which is adjusted to the desired level for oxidation, as will be explained hereinafter. In a preferred embodiment of the invention, voltage source 30 may be a conventional two and a half or three volt battery and potentiometer 32 preferably has a maximum resistance of no more than a few hundred ohms to provide a relatively low impedance sensing circuit between the electrodes. It will be appreciated that other suitable means for electrically connecting the electrodes may be used which allow a current flow, which current is caused by electrons generated at the sensing electrode at which an electrooxidation reaction occurs.

In a preferred embodiment, as shown in the drawings, the surface of the electrolyte is covered with a semipermeable membrane l7 through which the selected gas may diffuse, but which will prevent significant losses of electrolyte by evaporation or spillage. Preferably a membrane material, such as Teflon, having a high diffusion rate for the selected gas or gases and a lower diffusion rate for the interfering species can be employed to improve operation. However, in a simplified device the membrane may be eliminated and the electrolyte surface fully exposed to a gas. As the nitrogen oxide containing gas contacts the electrolyte surface,

molecules will diffuse into the electrolyte with the oxide concentration initially entering the electrolyte solution being proportional to their partial pressures or concentrations within the atmosphere. Thereafter, as molecules of NO and N0 (or 50 in solution in the electrolyte contact the surface of the sensing electrode 13, they become electrooxidized with the specific reaction depending on the type of electrolyte present and the material making up or relative potential of the counter electrode as will be more fully explained hereinafter. As the electrooxidation producing nitrate ions (NO (or sulfate ions [80,- occurs, a current generated between the sensing and counter electrodes is monitored by suitable means 15 which may include amplification equipment. Although the current will depend on the rate of oxide diffusion into the particular electrolyte, temperature and pressure variations, etc., it will be evident that the current in any event will be proportional to the concentration of atmospheric nitrogen oxides.

It is especially important that the dissolved oxide molecules present in the electrolyte are essentially confined to the portion of the electrolyte 11a between the sensing electrode 13 and the electrolyte surface. Significant diffusion of these oxide ions beyond the sensing electrode 13 and throughout the bulk of electrolyte 1lb will be prevented during transducer operation since essentially all of the molecules contacting the sensing electrode 13 will become immediately electrooxidized. Accordingly, as the nitrogen oxide concentration within the portion of electrolyte contacting the sensing electrode 13 is continually diminished, more molecules diffuse to that portion of electrolyte and thereafter are electrooxidized. Thus, further oxide diffusion into the portion of electrolyte llb between the counter and sensing electrodes is essentially prevented. lt will be evident that nitrogen oxides present at the counter electrode 12 would become directly oxidized with no resulting current flowing between the counter and sensing electrodes from the reaction.

As previously noted, the vessel may be open to the atmosphere or be constructed as shown in FIG. 1 whereby a cover 18 is present. Where such a cover is utilized, means for directing a gas into the sample gas space 16, such as a gas inlet 24 and outlet 25, are provided. The cover 18 may also be provided with a groove or slot 26 for seating a gasket or O-ring 27 which will complete the enclosure of the gas sample space 16 and confine the gas. The cover 18 and vessel 10 may additionally be provided with appropriate bore holes 28 through which bolts may be placed for securing the cover 18 to the vessel 10. Obviously, other means such as clamping devices and the like may also be used for this purpose. This type of construction is especially suited for directing gas streams such as automobile exhausts and the like to be analyzed.

It will be appreciated that the transducer method disclosed herein may be used for analysis of sulfur dioxide gases and nitrogen oxides by proper selection of counter electrodes and electrolyte compositions and by proper selection of the bias voltage.

The sensing electrode may consist of any noble metal which itself does not undergo electrochemical reaction within the electrolyte. Examples of suitable metals include gold, platinum, palladium, iridium and the like. The electrode itself may consist of a screen, foil, porous plaque or fabricated in such other suitable form as desired. In forming an electrode of such precious metals, as a practical matter it is often preferred to form a coating of the inert metal on relatively less expensive metallic substrate materials. Thus, for example, a sensing electrode consisting of a gold-plated copper or nickel expanded metal is found to be quite satisfactory. Further, it is preferred to fabricate this sensing electrode in a manner to expose a rather large electrode surface area to the electrolyte solution. Accordingly, fine screens or porous electrodes may be preferred.

The counter electrode consists of an electroactive material which is capable of being electroreduced when in contact with the electrolyte. Where the transducer is to act as an electrooxidant type sensor, i.e. where sulfur dioxide, nitric oxide and/or nitrogen dioxide are to be electrooxidized at the sensing electrode, the counter electrode must comprise a material which will utilize the electrons released in generating the nitrate and/or sulfate ions at the sensing electrode by the electrooxidation reaction. The counter electrode mate rial utilizes the electrons and itself is electroreduced to form an inactive material. The counter electrode composition of this invention which is capable of accepting the released electrons will be described more fully hereinafter.

Theoretically electrooxidation of diffused sulfur dioxide in an aqueous acid electrolyte solution at the sensing electrode is carried out at 0.17 volts (Stockholm Convention) relative to the standard hydrogen electrode and which polarity is positive relative to the standard hydrogen electrode. The acid electrolyte in this electrooxide type sensor is preferably dilute sulfuric acid, although other acids may be used. In the operation of the preferred embodiment shown and described herein, the counter electrode consists of an electroactive material employed in conjunction with external bias voltage source 30 and potentiometer 32, placed in series with the sensing circuit, to adjust the potential between the electrode to the desired level for oxidation.

Theoretically electrooxidation of diffused nitrogen dioxide in aqueous acid electrolyte solution at the sensing electrode is carried out at +0.80 volts (Stockholm Convention) relative to the standard hydrogen electrode, and which polarity is positive relative to the standard hydrogen electrode whereas nitric oxide electrooxidation occurs at about +0.96 and +1.03 volts. The acid electrolyte in this electrooxidant type sensor is preferably dilute sulfuric acid although other acid solutions may be used.

in the embodiment shown and described herein, the counter electrode consists of an electroactive material employed in conjunction with external bias voltage source 30 and potentiometer 32., placed in series in the sensing circuit, to adjust the potential between the electrode to the desired level for oxidation.

The electrooxidation reactions proceed readily in dilute acid electrolyte without significant interference by the presence of oxygen, carbon dioxide, carbon monoxide, water vapor, hydrocarbons, carbonyl compounds such as aldehydes and ketone in the diffused gaseous mixture. The presence of sulfur dioxide in the nitrogen oxide containing sample will oxidzie at a lower potential and interfere with the nitrogen oxide measurement. Although sulfur dioxide interference may not be serious in analysis of automobile exhausts, its presence in other gaseous samples and the concentrations thereof may readily be determined by the method of analysis set forth herein. Thus, the sulfur dioxide concentration is monitored without nitrogen oxide interference by selecting a suitable lower oxidation potential, and a higher oxidation potential is selected to monitor the combined sulfur dioxide and nitrogen oxide concentrations. Accordingly, the difference between the monitoring of sulfur dioxide exclusively and the monitoring of the combined gases is determined, taking into consideration variations of instrument response, etc., to yield the concentration of nitrogen oxide.

Thus, a transducer is provided according to the preferred embodiment of the invention shown and described herein having a bias voltage source for selectively adjusting the potential between the two electrodes to the potentials determined for oxidation of sulfur dioxide above +0.17 volts, oxidation of nitrogen dioxide above 0.80 volts or oxidation of nitric oxide above +0.96 volts.

Again, as in the case of the sensing electrodes, a number of different techniques for fabricating the counter electrode may be used. Thus, the compositions may be plated or otherwise impregnated onto inert materials in such a manner as to allow maximum exposure of the electroactive material to the electrolyte solution. Where the apparatus is to be used for relatively long periods of time, it may be desirable to cover the counter electrode with a suitable ion-exchange or ionselective membrane which will prevent the gradual buildup of dissolved metal ions originating from the counter electrode. The use of such a cover prevents possible changes of surface characteristics resulting from extensive buildup of ionic materials.

The gaseous mixture which is to be analyzed for nitrogen oxide (and/or sulfur dioxide) according to the present invention may be exposed to the electrolyte solution in which it will be diffused by any suitable manner. Where an open vessel is used, gaseous diffusion at the electrolyte surface will readily take place by mere exposure to the nitrogen oxide containing atmosphere. Where an enclosed vessel is preferred, a space between the vessel wall and the electrolyte surface into which space gases may be directed is necessary. The gaseous mixture may then be pumped or otherwise fed into the space continuously or intermittently as desired. As previously noted, in order to prevent extensive electrolyte evaporation, a semi-permeable membrane of an inert material which will not prevent or substantially impede gaseous diffusion of the nitrogen oxides into the electrolyte may be used to cover the surface of the electrolyte solution. For example, Teflon, polyethylene, polypropylene and the like are suitable where the particular material may be chosen for its relative impermeability to possible interfering gases. The membrane will also prevent loss of the electrolyte by spillage and will provide improved convenience since the device may be placed in any position during use or storage without sig nificant loss of electrolyte.

Monitoring of the current passing between the sensor electrode and the counter electrode may be accomplished by any suitable means. Although the current is directly proportional to the partial pressure of nitrogen oxides present in the atmosphere diffusing into the electrolyte solution due to the relatively low current intensity, appropriate electronic amplification will be useful. Further, equipment calibrated to read directly in parts-per-million nitrogen oxide is effective in continually monitoring the output voltage of the amplifier although other suitable means may be selected.

The counter electrode of this invention is a lead sulfate mixture which is prepared by mixing about 5.0 grams of reagent grade lead sulfate, 0.5 grams carbon black, and 0.2 grams polypropylene powder. This produces a lead sulfate mixture which is appropriate for use as a counter electrode of this invention and which will accept electrons generated by the oxidation reaction at the sensor electrode.

In one manner of constructing the counter electrode of this invention, approximately three grams of the lead sulfate mixture is spread gently in a cylindrical mold 1.5 inches in diameter by about 0.25 inches deep having removable top and bottom plates. A flat circular piece of platinum about 1.5 inches in diameter by about 0.004 inches thick and having a No. 80 mesh size is placed in intimate contact with the mixture and the remainder of the mixture is spread over the mesh. The mold is closed and the assembly pressurized to 3,000 psig at 110C. for five minutes in a hydraulic press. Thereafter, the mold is removed from the press and cooled.

In one embodiment of the invention, the sensor electrode consisted of a 3-inch diameter circular plate of gold. The electrodes were commonly wired to electronic current amplification equipment and were assembled in a plexiglass container to which an aqueous solution of 1N sulfuric acid was carefully added to avoid trapped airbubbles. The gold electrode was covered with a A mil thick Teflon membrane and the entire assembly made secure. Into the air space above the membrane was continually passed an atmospheric gaseous mixture containing sulfur dioxide, nitric oxide and nitrogen dioxide through an inlet tube extending from the exterior of the plexiglass container. The gas exited from the enclosure through a similar projection tube opposite the inlet tube. The voltage bias was adjusted to provide about +0.80 volts at the sensing electrode. Since the counter electrode reaction occurs at about 0.36 volts relative to the standard hydrogen electrode, the voltage bias was adjusted to about +1.16 volts. Oxidation of the diffused sulfur dioxide immediately took place at the sensing electrode producing negatively charged sulfate ions (SO to produce a current flow forreduction of the lead sulfate counter electrode composition to produce lead and sulfate ions. Specifically, the reactions at the sensing electrode and counter electrodes are as follows:

ZH O S0 2e 4H SO 0.17 volts PbSO, 2c Pb SO, 0.36 volts The current caused by the two simultaneous reactions was continually monitored while the sulfur dioxide concentration of the gaseous mixture entering the analyzer was changed. Initially the response time of the apparatus to indicate percent of the actual initial sulfur dioxide concentration of ppm was about 10 seconds. Thereafter, the sulfur dioxide concentration was changed to 50 ppm with the recovery time of the analyzer in registering the change in concentration being about 15 seconds.

Thereafter, the voltage bias was adjusted to about +1.55 volts to provide about +1.20 volts at the sensing electrode. Diffused nitrogen dioxide oxidation immediately took place at the sensor electrode producing negatively charged nitrate ions (NO with concomitant reduction of the lead sulfate counter electrode composition to produce lead and release sulfate ions by reactions similar to those previously described. The current caused by the simultaneous reactions was continuously monitored while the nitrogen oxide concentrations of the gaseous mixture entering the analyzer was changed. Initially, the response time of the apparatus to indicate 90 percent of the actual initial nitric oxide and nitrogen dioxide concentration of 100 ppm was about seconds. Thereafter, the total concentration of both oxides was changed to 50 ppm with the recovery time of the analyzer in registering the change in concentration being about seconds.

The invention has been described in preferred forms with particularity, but this is only by way of example. Various changes in construction and application may be made without departing from the spirit or scope of the invention.

What is claimed is: V

1. In a monitoring process for measuring the concentration of nitrogen oxides or of sulfur dioxide pollutants in a gas sample, the method comprising:

providing a confined volume of aqueous acid electrolyte having an active surface for receiving molecules from said gas sample to be dissolved in said electrolyte;

providing a sensing electrode immersed in said electrolyte adjacent the active surface consisting of a chemically inert material;

providing a counter electrode immersed in said electrolyte and spaced from the sensing electrode on the side opposite the active surface of the electrolyte consisting of a lead sulfate electroactive mixture capable of being electroreduced to metallic lead;

externally coupling said sensing electrode to said counter electrode;

maintaining said sensing electrode at a predetermined positive potential relative to the standard hydrogen reference electrode, said potential being above 0.17 volts for sulfur dioxide and above 1.03 volts for both sulfur dioxide and nitrogen oxides;

conveying the gaseous sample to introduce molecules of the gaseous sample at the active surface to be dissolved in said electrolyte at a rate proportional to the concentration of the pollutants in said sample,

said pollutants dissolved at the active surface of said electrolyte being electrooxidized by coming in contact with said sensing electrode to produce negatively charged ions and to release electrons, and said electroactive material of said counter electrode being electroreduced by said electrons to produce sulfate ions and chemically inactive lead at said counter electrode; and,

measuring the current flow in an external circuit coupled between said sensing; electrode and said counter electrode to indicate the concentrations of pollutants in said sample.

2. The method of claim 1 wherein said sensing electrode and said counter electrode are coupled through a current sensing device and a low impedance voltage bias source.

3. The method of claim 2 further comprising:

aittta aiaas i ens n le'ctrqds at. a p d mined positive potential of about 0.80 volts relative to the standard hydrogen reference electrode whereby an indication of the concentrations of sulfur dioxide in said sample is provided by said measuring of the current flow.

4. The method of claim 2 further comprising:

maintaining said sensing electrode at a predetermined positive potential of about 1.20 volts relative to the standard hydrogen reference electrode whereby an indication of the concentration of nitrogen oxides in said sample is provided by said measuring of the current flow.

5. The method of claim 1 further comprising:

covering said active surface with a selectively permeable membrane readily permeable to said pollutant molecules and relatively impermeable to said electrolyte whereby the pollutant molecules diffuse through said membrane to said active surface. 

2. The method of claim 1 wherein said sensing electrode and said counter electrode are coupled through a current sensing device and a low impedance voltage bias source.
 3. The method of claim 2 further comprising: maintaining said sensing electrode at a predetermined positive potential of about 0.80 volts relative to the standard hydrogen reference electrode whereby an indication of the concentrations of sulfur dioxide in said sample is provided by said measuring of the current flow.
 4. The method of claim 2 further comprising: maintaining said sensing electrode at a predetermined positive potential of about 1.20 volts relative to the standard hydrogen reference electrode whereby an indication of the concentration of nitrogen oxides in said sample is provided by said measuring of the current flow.
 5. The method of claim 1 further comprising: covering said active surface with a selectively permeable membrane readily permeable to said pollutant molecules and relatively impermeable to said electrolyte whereby the pollutant molecules diffuse through said membrane to said active surface. 